Re: [PATCH] mm/swapfile.c: select the swap device with default priority round robin

[Chris Li <chrisl@kernel.org>]

On Fri, Sep 26, 2025 at 8:31 AM Baoquan He <bhe@redhat.com> wrote:
> On 09/25/25 at 11:25am, Chris Li wrote:
> > Just curious, is setting to "-1" matches to kernel behavior before
> > a2468cc9bfdf, if not what is the behavior before a2468cc9bfdf.
>
> It should be like below. It's not a real output, I made the data to show
> what it looks like.
>
> # swapon
> NAME       TYPE      SIZE  USED PRIO
> /dev/zram0 partition  16G 15.8G   -1
> /dev/zram1 partition  16G    0B   -2
> /dev/zram2 partition  16G    0B   -3
> /dev/zram3 partition  16G    0B   -4
>
> I just apply this patch and set the priority to emulate the kerel
> behavirour before a2468cc9bfdf. In kernel before a2468cc9bfdf, it sets
> priority to swap device from -1 downwards. There's only one
> swap_avail_head plist for all CPUs. The behaviour is very much like below:
>
> [root@hp-dl385g10-03 ~]# swapon
> NAME       TYPE      SIZE  USED PRIO
> /dev/zram0 partition  16G    0B    0
> /dev/zram1 partition  16G    0B    1
> /dev/zram2 partition  16G    0B    2
> /dev/zram3 partition  16G 14.3G    3

I see, in that case it is not a simple revert. It is revert plus a
change to make the default to use round robin. I suggest you split the
patch into two parts, one is the conceptual clean revert, back to the
pre a2468cc9bfdf without behavior change. Then the second one is
changed to the round robin. That will make your behavior change more
obvious.

I suspect that even pre a2468cc9bfdf, the round robin might outperform
per node priority already. Which means there exists a much simpler
solution all alone. Even though I am curious, I am not demanding the
answer from you. Your test data against the latest kernel, which shows
great performance improvement is good enough. We don't have to test
that really old kernel.

Chris